1. Field of the Invention
This invention relates to an apparatus which detects the shape of a target carried on a conveyor.
2. Description of the Related Art
FIG. 17 is a drawing showing the construction of a conventional radar-type object shape detector.
FIG. 18 is a descriptive diagram showing the positional relation between a radar device and an object in this conventional detector. In the figures, a transmitter 25 emits a continuous waveform signal with a fixed transmitting frequency. A transmitting antenna 26 forms a narrow beam having a sufficiently high angular resolution to detect the shape of a target 11. The transmitting antenna 26 is driven by an antenna driver 29 so as to cause the beam 32 to sweep in a vertical direction at an interval equal to the beam width. Each time the target 11 moves horizontally by the same amount as the width of the beam 32 in a horizontal direction, the beam 23 is made to sweep progressively in a vertical direction. Due to this sweep of the beam, reflected signals are obtained from the whole surface of the target 11.
A receiving antenna 28 receives a reflected signal beam 33, this beam being of the same shape as the transmitting beam 32. The receiving antenna 28 is driven by the antenna driver 29 so that it sweeps across the reflected beam 33 in the same way as the transmitting beam 32 is made to sweep. The reflected signals from the target 11 which are received by the receiving antenna 28, are amplified and detected by a receiver 30. The receiver 30 outputs video signals obtained by this detection to a display 31 in synchronism with the sweep of the transmitting beam generated by the antenna driver 29.
When the display 31 receives these video signals, it also receives a location signal showing the distance the target was carried by a conveyor 27, and an antenna angle signal output by the antenna driver 29. The display 31 computes the cooridnates of the target in a horizontal direction from the location signal, and the coordinates of the target in a vertical direction from the antenna angle signal, and displays the aforementioned video signal based on these coordinates. The display 31 thereby forms an image of the target. In this way, a conventional radar-type object shape detector was able to detect and display the shape of a moving target.
Due to the aforesaid construction, this radar-type object shape detector had to form a narrow beam with a high angular resolution in order to detect the shape of the target in a desired degree of detail. However in radar devices, the radar beam width is inversely proportional to the aperture size of the antenna. Hence, in order to form a narrow beam and improve the resolution of a conventional detector, the aperture size of the transmitting and receiving antennas had to be increased, which made the apparatus too bulky. As an example, to form a beam of wavelength=3 mm and beam width=0.1.degree., the antenna was required to have an aperture size of approximately 2.1 m.
This invention aims to overcome the above problem. It therefore aims to provide a radar-type object shape detector which can detect the shape of a target with high resolution even using a compact antenna of small aperture size and consequently, a large beam width.